Nuclear fuel bundles for boiling water reactors are usually surrounded by a fuel box. The main functions of the fuel box are to provide mechanical stability to the bundle and to conduct the cooling water so that adequate cooling of the fuel bundle in all its parts is obtained.
The fuel box has a square cross section and surrounds the bundle along all of its length. At the bottom of the fuel box a transition piece is attached, and at the bottom thereof there is a guide intended to be arranged in the assembly supporting plate of the reactor. The box may also be provided with an inner, usually cruciform structure which axially divides the fuel bundle into four sub-bundles.
The shape accuracy of the fuel box is of the utmost importance for its function. It is also important for the box to have good corrosion resistance during reactor operation. The box has large surfaces in contact with the reactor coolant. Flaking corrosion products should not form on the box surfaces since these products may spread radioactivity to various systems in the reactor. There should also be a good margin with respect to weakening of the box wall caused by the metal being transformed into oxide.
During manufacturing of a fuel box, thin rectangular plates of a zirconium alloy constitute the starting material. Zirconium alloys widely used in nuclear reactors are Zircaloy-2 and Zircaloy-4. Zircaloy-2 contains 1.2 to 1.7% tin, 0.07 to 0.20% iron, 0.05 to 0.15% chromium, 0.03 to 0.08% nickel, 0.09 to 0.16% O and Zircaloy-4 contains largely the same alloying elements but lacks nickel and contains somewhat more iron, 0.018 to 0.24%. Also other zirconium alloys for reactor purposes such as, for example, a zirconium-base alloy containing about 1% tin, about 1% niobium and about 0.2% iron, or a zirconium alloy containing about 1% niobium, 1% tin, 0.5% iron and 0.2% chromium may be used as starting material. The alloys comprise incidental impurities, normally in the range of 500 to 1500 ppm.
The box is manufactured by bending two plates into U-shape. The bending is carried out with a conventional method and may be preceded by a heat treatment of the plate to increase its ductility. Two U-shaped plates are turned so as to face each other and are welded together along the folded-up parts of the plates, so as to obtain a box with a square cross section. The shaping of the box into the finished dimension is made by heat-treating the box on a device in a conventional manner.
To improve the corrosion properties of the material, it is known, according to GB 1 537 930 , to heat the material to a temperature exceeding 900.degree. C. so that a phase transformation occurs in the material from hexagonal alpha phase to cubic beta phase, and thereafter to cool the material rapidly, so-called beta quenching. Phase transformation occurs at about 870.degree. C., and above about 930.degree. C. the material is completely transformed into beta phase. The temperature may vary somewhat depending on what alloying elements the zirconium contains, however all zirconium alloys for reactor purposes are low alloy elements so the variation in phase transformation temperature is relatively small.
British patent GB 1 537 930 describes that the plate is to be heated to a temperature of at least 900.degree. C. by allowing the temperature rise from 500.degree. C. to the desired heat-treatment temperature and the heat treatment is to take at most 60 seconds, whereafter the plate is to be cooled at least 200.degree. C. in at most 60 seconds. During cooling of the plate, the material forms an acicular structure, so called Widmanstatten structure.
U.S. Pat. No. 4,238,251 describes heat treatment of nuclear fuel components for improving the corrosion resistance in boiling water reactors.
A fuel box in a finished or almost finished form is heat-treated at a temperature such that an incipient phase transformation from alpha to beta takes place, whereupon the box is rapidly cooled to about 700.degree. C. This heat treatment, so-called beta quenching, brings about a segregation of intermetallic particles in a two-dimensional pattern. The heat treatment is primarily to take place at a temperature higher than 965.degree. C. and should not occur at a temperature exceeding 1100.degree. C. since this is too energy-demanding without providing structural advantages compared with heat treatments at lower temperatures. The box should be maintained at the heat-treatment temperature for about 3 to 30 seconds and then be quenched to a temperature lower than 700.degree. C. at a rate of about 200.degree. C. per second.
U.S. Pat. No. 5,361,282 also describes a heat treatment of zirconium plates for fuel boxes to achieve good corrosion resistance in a boiling-water reactor environment. The heat treatment means that the material is heated to 980.degree. C. to 1120.degree. C. and is kept at the heat-treatment temperature for 0.25 seconds to 30 minutes, whereafter the material is cooled down to a temperature lower than 815.degree. C. at a cooling rate of 6-240.degree. C. per second. The beta quenching heat treatment leads to a random distribution of the crystals in the hexagonal alpha structure. A random distribution of crystals will decrease the tendency of the fuel box to bow in reactor service.
A problem with the heat treatments which are conventionally used for improving the corrosion resistance of the box material in a boiling-water reactor environment is that the it is difficult to bend the box plate to the proper shape after the heat treatment. Because of the structure which is formed after the heating to beta phase with a subsequent rapid cooling to alpha phase, cracks easily occur in the material when this is to be bent into U-shape before the manufacture of a fuel box. Nor does a preliminary heat treatment of the material to increase the ductility give sufficiently good results for the material to be capable of being bent without the risk of cracking. In addition, the possibility of preheating the material is limited by the fact that this heat treatment may deteriorate the corrosion properties of the material.